The invention relates to a liquid level meter for measuring a level of a liquid in a container, in which electromagnetic signals are transmitted in the direction of the surface of the filled material, and their echo is received. A time of flight required by a signal for the path to the surface of the filled material and back is ascertained, and the level is determined therefrom.
Liquid level meters operating with electromagnetic signals can be used in a multiplicity of applications, both in storage and in the processing industry, for example in chemistry, in the food industry and in the oil industry.
Liquid level meters operating with microwaves, inter alia, are commercially available. Said meters are divided into two classes: a first class, in which the microwaves are transmitted by means of an antenna in the direction of the filled material, are reflected at the surface of the filled material and, after a time of flight which is a function of distance, are subsequently received again; and a second class, in which the microwaves are guided along a waveguide in the direction of the filled material and are reflected at the surface of the filled material because of the jump in impedance there, and the reflected waves are guided out of the container again along the waveguide.
In this case, the waveguide can be provided both by a single conductor and by two or more conductors which are arranged parallel to one another and extend downwards into the container from a point above the highest level to be measured.
DE-U 94 21 870 specifies a liquid level meter for measuring a level of a liquid in a container, which comprises
an electronic circuit
which generates electromagnetic signals in operation, PA1 which projects into the container, PA1 which transmits the signals from the circuit to the conductor, PA1 which conductor leads the signals into the container and leads out signals reflected at a surface of the filled material, and PA1 which receives reflected signals, PA1 which determines a time of flight of the electromagnetic signals and PA1 which ascertains the level from the time of flight. PA1 which generates electromagnetic signals in operation, PA1 which projects into the container, PA1 along which a float moves along with the level, PA1 which transmits the signals from the circuit to the conductor, PA1 which receives the reflected signal, PA1 which determines a time of flight of the electromagnetic signals and PA1 which ascertains the level from the time of flight.
a conductor
an injector
a receiving and evaluating circuit
An electronic circuit for generating the electromagnetic signals, and a receiving and evaluating circuit, are described, for example, in U.S. Pat. No. 5,841,666.
In the case of the liquid level meter in accordance with DE-U 94 21 870, the signals are short electromagnetic pulses which are reflected at the surface of the filled material. The proportion of the signals which is reflected depends in this case on the difference between the dielectric constants of the filled material and the medium located above the filled material.
If the difference is large, the measurement effect is also marked, and the time of flight can be determined acceptably. However, if the difference is slight, the reflection originating at the surface of the filled material can no longer be readily ascertained. In such applications, the described liquid level meter is not functional. An example for such a problematic application is the measurement of a level in an oil-filled container. Oil has a dielectric constant of approximately 2, while the air typically located above the oil has a dielectric constant of 1.
This effect also occurs in liquid level measurement by means of free microwaves which are transmitted in the direction of the surface of the filled material and are reflected at the surface, their echo signal being evaluated in order to determine the level.
A description is given in DE-A 44 19 462 of a liquid level meter which operates with microwaves and in the case of which microwaves are transmitted into a stilling well projecting into a container. In this case, the stilling well serves as a waveguide for the microwaves. It is specified for applications in which the filled material has a low dielectric constant in order to provide a float with a strongly reflecting surface in the stilling well.
A stilling well is a large bulky and therefore cumbersome component, and therefore cannot be used when only a little space is available and/or an opening provided for a liquid level meter has too small a diameter. The installation of a stilling well is complicated and expensive. Moreover, in the case, for example, of applications for which the stilling well comes into contact with viscous and/or sticky media, regular cleaning may be required for the float to remain freely movable.